Electroless cobalt plating bath



Dec. 17, 1968 M. P. MAKOWSKI ELECTROLESS COBALT PLATING BATH Filed Jan. 13, 1965 O w 0 9 H o O S 8 Mm m A A H T Q O YG EHN .0 5 o 4 n o O O O o 03 o O O O A 3 2 J O O O 0 I301 Ema wdE ZOrCMOQmQ .F EQOU TEMPERATURE C v vATTORNEY United States Patent 3,416,955 ELECTROLESS COBALT PLATING BATH Mieczyslaw Paul Makowski, Willowick, Ohio, assignor to Clevite Corporation, a corporation of Ohio Filed Jan; 13, 1965, Ser. No. 425,277 13 Claims. (Cl. 117-160) ABSTRACT OF THE DISCLOSURE An electroless plating bath for depositing cobalt in which the water base bath contains a hydrazine base reducing agent, a solution of ethylenediamine derivatives for complexing the cobalt ions, an alkali agent for controlling the hydrogen ion concentration.

This invention relates to plating by chemical reduction and, more particularly, to a method and bath for plating metallic and non-metallic surfaces with a coating of cobalt.

Electroless plating of cobalt by chemical reduction while of relatively recent origin, is well known and used for numerous applications. In the conventional process a bath containing either sodium hypophosphite or a boron containing reducing agent is employed. The use of such reducing agents causes the co-deposition of phosphorus or boron impurities in the final cobalt plating. Applications requiring a very high degree of cobalt purity cannot employ this type of electroless plating bath [for this very reason.

In order to circumvent the above mentioned difficulty it has now been suggested to use hydrazine as the reductant. Such a solution decomposes, however, quite readily and, therefore, must be used in conjunction with a compatible complexing agent to stabilize the bath for a period of time.

It has been discovered that a cobalt plating bath solution containing hydrazine as a reducing agent and EDTA, or other derivatives of ethylenediamine, as a complexing agent provides a bath which appears to be superior, for the above mentioned purpose, than any other heretofore known. The bath establishes a very uniform rate of metal deposition. The bath is uniquely stable. While a conventional cobalt bath, for instance with hypophosphite as a reducing agent, can be stored for a few days only, the cobalt bath disclosed herein has been stored for over one year and at that time showed no signs of deterioration. The maximum period for storing the bath is as yet unknown. A further advantage of this bath is that it will not decompose on being overheated. With the Brenner bath care must be exercised to prevent accidental over-heating, as it will render the bath useless. The hydrazine-EDTA complex cobalt solution can be boiled yet will not decompose. Finally, the solution has shown to be satisfactory for plating cobalt on various substrates such as steel, copper, smooth nickel, porous nickel, glass and various plastics. In the case of non-metals the customary sensitizing or palladium seeding procedure was utilized satisfactorily. The bath was used repeatedly merely by maintaining a sufficient cobalt concentration. It appears that the general formulation of the bath may also, under certain conditions, be helpful for plating nickel from nickel chloride or sulfate solutions.

One aspect of the present invention resides in the provision of an electroless plating bath for depositing cobalt upon a body or surface. The bath is composed basically of the following ingredients in addition to water; a source of cobalt ions, either in the form of cobalt sulfate heptahydrate, cobalt chloride hexahydrate, or a cobalt nitrate solution; a hydrazine base reducing agent and a solutior of EDTA or certain hereinafter described derivatives 01 ethylenediamine, for complexing the cobalt ions and the solution of hydrazine at room temperature while permitting the complex to react at elevated temperatures. A strong alkali solution is added for controlling the hydrogen ion concentration in the bath to maintain the pH factor therein in the alkaline range.

Another aspect of this invention is to provide a methoc' for utilizing the above described bath in the most advantageous manner. The bath is heated from room temperature to a temperature range of about 60 to C. tc effect a depositing rate of the cobalt of about .04 to 0.6( mil per hour.

It is' therefore the primary object of this invention tc provide an improved plating bath for depositing pure cobalt to metallic or non-metallic surfaces.

It is a further and more specific object of this inventior to provide an improved plating bath for depositing pure cobalt to metallic or non-metallic surfaces.

It is'a further and more specific object of this invention to provide an improved plating bath for depositing pure cobalt which when accidentally overheated does not decompose.

It is another object of this invention to provide a bath of the type hereinabove referred to which can be stored for relatively long periods of time without any danger of decomposition.

It is a further object of this invention to provide a bath of the type here under consideration which will provide a superior and extremely uniform rate of deposition.

It is still another object of this invention to provide a method to utilize the electroless cobalt plating bath under conditions which permit optimum results.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

The single figure illustrates the relative amounts of plating deposited with changes in temperature.

The preliminary treatment of the body prior to plating is conventional. The surface is cleaned and care is taken that the same is free of grease. In the case of non-metallic bodies, a conventional chemical cleaning treatment is applied.

The following are examples of general and more specific cobalt bath solutions in accordance with this invention in which water constitutes the base solution.

EXAMPLE 1 To a solution of cobalt sulfate heptahydrate or cobalt chloride hexahydrate is added 0.15 to 0.30 mole per liter of a complexing agent of the ethylenediamine type and 'a reducing agent of 2.0 to 4.0 mole per liter hydrazine. Tests have proven sodium salts of N-ethylenediamine triacetic acid or ethylene-diamine-tetraacetic acid to be satisfactory. To this mixture is added a strong alkali agent, for instance 0.5 to 3.0 mole per liter of sodium hydroxide, to control the hydrogen ion concentration in the bath to a degree permitting the pH value to be in the alkaline range. The preferred pH value for the above named complexing agents is to 11, this value is readily obtainable.

The deposition rate of the cobalt upon the body which is suitably suspended in the bath, ranges, for the formulations of Example 1, from about 0.04 to0.60 mil per hour. This is based on a useful temperature range of about 60 to 95 C. and variations in the cobalt ion concentration. The diagram shown in the drawing'indicates, generally, the relationship between temperature and rate of deposition for one of the herein disclosed formulations.

In the following example a specific concentration of cobalt ions is indicated together with preferred quantities of the bath constituents. A reasonable amount of deviation from these preferred quantities is permissible.

EXAMPLE 2 The cobalt ion solution consists of about grams per liter of cobalt sulfate heptahydrate, to which is added a complexing agent consisting of approximately 80 milliliters per liter of a 2 molar solution of trisodium salt of N-hydroxyethyl ethylenediamine triacetic acid and a reducing agent of about 125 milliliters per liter of about 65% hydrazine base solution. A pH controlling agent of about 100 grams per liter sodium hydroxide is also added to the bath.

The bath will remain stable at room temperature and react in the elevated temperature range of 60 to 95 C. In this temperature range the bath will deposit the cobalt very uniformly at the rate of up to 0.6 mil per hour.

EXAMPLE 3 Herein the cobalt ion solution consists of about grams per liter of cobalt chloride hexahydrate, to which is added a complexing agent of about 80 grams per liter of tetrasodium salt of ethylenediamine tetraacetic acid (EDTA) to stabilize the solution and a reducing agent of approximately 200 milliliters per liter of an approximately 65% hydrazine base solution. An amount of 20 grams per liter of sodium hydroxide is added to establish and maintain a pH value in the alkaline range.

As in the Examples 1 and 2, the cobalt ion reacts and establishes a plate formation at about to 95 C. Tests, however, have shown that with the bath of Example 3, optimum plating uniformity is obtained by maintaining the bath temperature between 80 to 90 C. Under these conditions deposition occurs at the rate of .04 to 0.25 mil per hour.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed, therefore, in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A water base bath for chemically depositing cobalt upon a body comprising: a source of cobalt ions in said bath having at least a minimum concentration for effecting a deposit of a coherent cobalt layer; a solution of hydrazine in an amount sufficient to establish a cobalt reduction; a strong alkali agent for controlling hydrogen ion concentration in the bath and effective for maintaining the pH therein in an alkaline range wherein the cobalt complex remains in stable solution; and a solution of ethylenediamine derivatives having nitrogen and oxygen donor atoms, said last mentioned solution being effective for complexing said cobalt ions and said solution of hydrazine at room temperature, the resulting complex reacting at relatively elevated temperatures.

2. A bath according to claim 1, wherein said derivative is a sodium salt of N-hydroxyethyl ethylenediamine triacetic acid.

3. A bath according to claim 1, wherein said derivative is a sodium salt of ethylenediamine tetraacetic acid.

4. A bath according to claim 1, wherein said source of cobalt is composed of a cobalt nitrate, cobalt sulfate or cobalt chloride base solution.

5. A water base bath for chemically depositing cobalt upon a body comprising: a source of cobalt ions in said bath having at least a minimum concentration for effecting a deposit of a coherent cobalt layer; a reducing agent containing 2.0 to 4.0 mole per liter of hydrazine a solution of 0.15 to 0.30 mole per liter of a complexing agent selected from a group of sodium salts consisting of N-hydroxyethyl ethylenediamine triacetate and ethylenediamine tetraacetate and derivatives thereof; and a strong alkali agent for controlling the hydrogen ion concentration in the bath for maintaining the pH therein in an alkaline range wherein the cobalt complex remains in stable solution.

6. A bath according to claim 5, wherein said agent consists of 0.5 to 3.0 mole per liter of sodium hydroxide.

7. A bath according to claim 5, wherein said source of cobalt includes a solution of 20 grams perliter cobalt sulfate heptahydrate.

8. A bath according to claim 5, wherein said source of cobalt includes a solution of 30 grams per liter cobalt chloride hexahydrate.

9. A water base bath for chemically depositing cobalt upon a solid body comprising: a source of cobalt ions consisting of about 20 grams per liter cobalt sulfate heptahydrate; a complexing agent consisting of about milliliters per liter of 2 molar solution of trisodium salt of N-hydroxyethyl ethylene-diamine triacetic acid; a pH controlling agent consisting of about 100 grams per liter sodium hydroxide; and a reducing agent consisting of approximately 125 milliliters per liter .of about 65% hydrazine base solution.

10. A water base bath for chemically depositing cobalt upon a solid body comprising: a source of cobalt ion consisting of about 30 grams per liter cobalt chloride hexahydrate; a complexing agent consisting of about 80 grams per liter tetrasodium salt of ethylenediamine tetraacetic acid; a pH controlling agent consisting of about 20 grams per liter sodium hydroxide; and a reducing agent of about 200 milliliters per liter of an approximately 65% hydrazine base solution.

11. A method for chemically depositing cobalt upon a body comprising the steps of: contacting the body with a bath composed of an aqueous solution of cobalt ion salt having at least a minimum concentration for effecting a deposit of a coherent cobalt layer upon said body, a solution of hydrazine in an amount sufficient to function as a cobalt reducing agent, a solution of ethylenediamine derivatives having nitrogen and oxygen donor atoms effective for complexing the bath, a strong alkali solution for controlling the concentration of hy drogen ion in the bath and effective for maintaining the pH in an alkaline range wherein the cobalt complex remains in stable solution; and heating the bath to a temperature range of about 60 to C. to effect a depositing rate of about 0.04 to 0.60 mil per hour.

12. A method for chemically depositing cobalt upon a body comprising the steps of: contacting the body with a bath composed of an aqueous solution of about 20 grams per liter cobalt sulfate heptahydrate, 80 milliliters per liter of a 2 molar solution of trisodium salt of N-hydroxyethyl ethylenediamine triacetic acid, a pH controlling agent of about grams per liter sodium hydroxide, and about milliliters per liter of about 65 hydrazine base solution; and maintaining the temperature of the bath at about 60 to 95 C. to effect a depositing rate of up to 0.6 mil per hour.

13. A method for chemically depositing cobalt upon a body comprising the steps of: contacting said body with a bath composed of an aqueous solution of about 30 grams per liter cobalt chloride hexahydrate, about 80 grams per liter of tetrasodium salt of ethylenediamine 3,416,955 6 tetraacetic acid, a pH controlling agent of about grams FOREIGN PATENTS per liter sodium hydroxide, a reducing agent of about 234,020 6/1951 Australia 200 milliliters per liter of about hydrazine base solution; and maintaining the temperature of the bath at about OTHER REFERENCES to C. for obtaining a depositing rate of about 5 Narcus, Metal Finishing, March 1952, P- 56 .04 to 0.25 mil per hour. TS200M5 87.

Sau'bestre, Metal Finishing, July 1962, p. 51 References Cited TS200M587.

Wein, The Glass Industry, September 1959, 476 UNITED STATES PATENTS l0 TP2O0M587 P 2,430,581 11/1947 Pessel 117-160 2,999,770 9/ 1961 Gutzeit 117-130 RALPH S. KENDALL, Primary Examinuer. 3,015,858 1/1962 Hendricks 11735X 3,024,134 3/1962 Nixon et a1 117 US 3,198,659 8/1965 Levy 117 130 15 ;1 1 

